Seamless tubular artificial casings are predominantly used in the production of cooked-meat sausages and scalded-emulsion sausages. Conventional casings have usually a 3- to 5-layer structure, wherein generally at least one of these layers comprises aliphatic polyamide and one comprises polyolefinic materials having water vapor-barrier character. Usually, the casings are biaxially drawn. In the case of the polyamide (PA) situated in the solid, the drawing leads to molecular orienting and to strain-induced crystallization. These effects produce a considerable increase in strength of the PA layers and a thermal shrinkage capacity of the casings, which is usually desirable in the application.
Since the biaxial drawing process in the case of tubular films always proceeds with the aid of free-standing stretching bubbles which are filled with gas or fluid cushions (termed the double-bubble process), the process cannot be controlled mechanically (from the outside). In principle there is always the risk of fluctuations of the bubble diameter (what are termed pulsations) up to sporadic overstretching and bursting of the stretching bubble. Since, among the polymer types conventionally used, only PA experiences significant orienting on drawing, the forces for stabilizing the stretching bubble also mainly proceed from this (progressive recovery forces).
Optimum bubble stability is only achievable when aliphatic PA types which crystallize moderately rapidly are used. Types which have proved to be particularly suitable are nylon 6, nylon 6/66 and also mixtures of these. Admixtures of further materials (for example other PA types, other polymers, pigments and other industrially desirable additives) are possible, but always have a tendency to destabilize the stretching bubble.
Frequently, demands are made on a food casing which cannot all be met with casing structures whose “supporting” component is a layer of nylon 6 (or of a mixture of nylon 6 and nylon 6/66).
The multilayer casings available on the market contain an outer main layer of nylon 6 and/or nylon 6/66, followed by thinner layers which do not form significant orienting on drawing. These are, for example, polyethylene layers. The inner surface is frequently formed by a further relatively thin PA layer. The thickness of the outer layer makes up usually 50 to 70% of the total film thickness and is therefore decisive for the stability of the stretching bubble and also for the mechanical strength of the casing.
Such casing structures, however, are frequently not found to be optimal in terms of performance. The use as sausage casing demands on the one hand high linear strength of the film and on extension the best possible elastic recovery, i.e. low residual extension. This may best be achieved by incorporating the thickest possible layer of nylon 6 into the structure. On the other hand, as far as possible soft-tough mechanics of the structure are required, in order that the casing, during end-processing and during stuffing, withstands the mechanical buckling loads as occur on high-speed modern systems. For this, structures having low-crystalline aliphatic copolyamides such as nylon 6/66 would be considerably more suitable. However, the aliphatic copolyamides, in the drawn state, do not approach nylon 6 with respect to strength and elastic recovery.
Surface layers of aliphatic copolyamides have a further disadvantage: owing to the low crystallinity of the polymers, surfaces lying one on top of another have a strong tendency to stick to one another. Sticking together can occur in the case of rolls wound up under standard tension, since the casings, during storage, develop a slight longitudinal shrinkage. Under extreme storage conditions, i.e. when the casings are stored at elevated temperature and high humidity, the adhesion tendency of the copolyamides increases further so that the risk of sticking together increases. Accordingly it would be advantageous to use copolyamide in an internal layer, or to cover it by a further layer, for example composed of nylon 6.
A food casing having two adjacent PA layers is described in EP-A 603 678. The second PA layer, which is covered by a PA layer surface, contains an inorganic and/or organic mixing additive. The object was to incorporate industrially necessary mixing additives into the casing structure and in this case to avoid the known production difficulties and disadvantages to quality which can occur in the case of additive-containing PA layers. The publication does not imply any effect on the relationships between linear strength and toughness or the like.
A further disadvantage in terms of performance of the conventional multilayer casings is their restricted printability. Food casings are frequently now used in printed form, in which case the printing is usually intended to proceed using physiologically and ecologically advantageous aqueous dispersion inks. The physical adhesion of such ink systems to PA is frequently inadequate in practice; in particular in the case of use as sausage casing, where high mechanical and climatic changing stresses occur, flaking off of ink layers can occur. Chemical anchoring is possible in principle in the case of most dispersion ink systems, in particular in the case of 2-component systems, but requires the presence of sufficiently reactive chemical groups in the film surface.
A surface composed of pure PA contains only very few reactive groups (end groups of the PA chain molecules). It would therefore be desirable to introduce additional reactive groups into the PA matrix, for example by blending in a further COOH-functional polymer.
Single layer sausage casings composed of mixtures containing PA and in proportion an acid-modified polyolefin are already disclosed by EP-B 550 833. As an example according to the invention, a casing composed of 79.6% by weight of nylon 6, 10% by weight of nylon 6-I/6-T, 10% by weight of an ethylene/methacrylic acid copolymer and 0.4% by weight of mica is specified therein. Foreign polymers such as ethylene/methacrylic acid copolymers, however, are not molecularly compatible with PA and in appropriate mixtures form a disperse phase. With increasing mixture fraction, the disperse phase can increasingly interfere with the coherence of the coherent PA phase which can lead to the loss of the general mechanical properties of the polyamide. This is expressed in particular in a drastic decrease of the bubble stability during drawing. In the case of casing structures which predominantly or entirely comprise such a mixture, therefore the fraction of foreign polymer must remain limited, i.e. any high concentration of reactive groups cannot be achieved. It would be desirable to be able to combine a casing surface which has been modified with reactive groups and is correspondingly readily printable with the better processing and mechanical properties of pure PA.